Method for using an alternate pressure viscometer

ABSTRACT

A method for determining the viscosity and relative change of viscosity of a fluid over plural shear rates caused by a decreasing or increasing pressure differential resulting from fluid flow to a defined chamber in a capillary system. The flow of liquid through the capillary restriction, the pressure variation rate and known dimensions of the system can be used typically by a processor to determine a rheological property of a fluid.

FIELD OF THE INVENTION

The present invention is directed to a method for determining theviscosity and relative change of viscosity of a fluid over plural shearrates caused by a decreasing or increasing pressure differentialresulting from fluid flow to a defined chamber in a capillary systemusing an alternate pressure viscometer device. The flow of liquidthrough the capillary restriction, the pressure variation rate and knowndimensions of the system can be related, typically by a processor, fordetermining a rheological property of a fluid. More particularly, theprocess can monitor the relative increase of apparent viscosity withregard to oxidative thickening.

BACKGROUND OF THE INVENTION

Rheology is branch of physics dealing with the deformation and flow ofmatter. It is particularly concerned with the properties of matter thatdetermine its behavior when a force is exerted on it. Thus, it isconcerned with the study of the change in form and flow of matter,embracing viscosity, elasticity and plasticity. The present applicationis directed to the subset of fluid dynamics concerned with the flow offluids, primarily liquids in Newtonian and non-Newtonian regimes.Rheological relationships can provide a direct assessment ofprocessability, are useful for monitoring and controlling a process, area sensitive method for material characterization (such as changes to themolecular weight), and useful for following the course of a chemicalreaction or changes to a fluid in simulated conditions. Rheologicalmeasurements allow the study of chemical, mechanical, thermal effects,effects of additives, or the course of reaction byproducts. Allmeasurements of viscosity involve imparting motion to a fluid andobserving the resulting deformation of that fluid.

Viscosity is a physical property that characterizes the flow resistanceof a fluid. It has been defined as a measure of the internal friction ofa fluid where the friction becomes apparent when a layer of fluid ismade to move in relation to another layer. It is the resistanceexperienced by one portion of a material moving over another portion ofthe material. Viscosity is commonly used to characterize petroleumfluids, such as fuels and lubricants, and often they are specified inthe trading and classification of petroleum products. Kinematicviscosity for petroleum products is commonly measured in a capillaryviscometer by a standard method such as the ASTM D445 standard. The ASTMD445 standard involves measuring the time for a fixed amount of liquidto flow under gravity through a calibrated glass capillary under areproducible driving head and a closely controlled temperature. Inpractice, this method has some challenges due to size limits of theapparatus, due to geometry, the relative sample size, and difficulty inchanging shear rates. In addition, the calibrated glass tubes arefragile, difficult to clean, and relatively expensive. Therefore, it isundesirable to use a capillary viscometer for samples which would tendto diminish the repeatability of the capillary tube for example bycoating the tubes, since these tubes would need to be removed andcleaned or disposed of prior to reuse. Changing glass capillary tubes inthe ASTM D445 standard is a cumbersome and delicate procedure with aprocess delay since the replacement glass capillary and temperature bathmust come to equilibrium.

Viscometers commonly are separated into three main types: Capillary,rotational and moving body. Most of these viscometers can produceviscosity measurements at a specified constant shear rate. Therefore, inorder to measure the viscosity over a range of shear rates, one needs torepeat the measurement by changing the parameters (such as height,capillary tube dimensions) for capillary tube viscometers, by changingthe rotating speed of the cone or cup in rotating viscometers, orchanging the density of the falling object in the moving bodyviscometer.

The capillary tube viscometer has been principally defined by theHagen-Poiseuille Equation especially for Newtonain fluids. In aNewtonian fluid the shear stress is proportional to the shear rate, andthe proportionality constant is called the viscosity. To measureviscosity with a capillary tube viscometer, the pressure drop and flowrate are independently measured and correlated to some standard fluid ofknown viscosity. The three general types of glass capillary viscometersmost frequently used include the modified Ostwald types for transparentliquids (Cannon-Fenske routine), the suspended level type fortransparent liquids (Cannon-Ubbelohde types) and the transverse-flow fortransparent and opaque liquids (British Standard BS U-tube reverseflow). While there are precise instructions for operating each of theabove capillary viscometers, generally all follow the same set of basicsteps. The test sample is inserted into the viscometer and temperaturecontrolled. After reaching test temperature the test sample is allowedto flow under gravity past two timing marks with time recorded on thecalibrated capillary tube. Thus, the driving force is the hydrostatichead of the test liquid. The viscosity is calculated as a product of theflow time and the calibration constant. External pressure can be appliedto many of the capillary viscometers to increase the range ofmeasurement to enable the study of non-Newtonian behavior.

SUMMARY OF THE INVENTION

The present invention is directed in part to a method for determiningthe rheological property of a fluid, such as viscosity and relativechange of viscosity of a fluid, over a plurality of shear rates causedby a decreasing or increasing pressure differential resulting from fluidflow to a defined chamber in a capillary system using an alternatepressure viscometer device. The flow of liquid through the capillaryrestriction, the pressure variation rate and known dimensions of thesystem can be related, typically by a processor, for determining arheological property of a fluid. More particularly, the process canmonitor the relative increase of apparent viscosity with regard tooxidative thickening. Accordingly, disclosed is a method for screeningor determining a rheological property of fluid comprising:

-   -   a) providing a fluid sample to a reservoir placed in a        thermostatic control system;    -   b) placing a capillary in fluid communication with the fluid        sample, wherein the capillary has a first end and a second end        with a substantially uniform diameter over a predetermined        length, the first end being submerged in the fluid sample to be        measured, the second end attached to a manifold having at least        one selectable valve, the capillary together with the manifold        and the at least one selectable valve define a chamber of        predetermined volume;    -   c) actuating at least one selectable valve attached to the        manifold to allow a gas to enter into and pass through the        manifold and capillary;    -   d) inducing the sample into the capillary by rapidly generating        a dynamic differential pressure in the chamber thus allowing the        sample to flow from the reservoir through the capillary;    -   e) detecting pressure change of the chamber as a result of the        fluid flow; and    -   f) relating the rate of pressure change to a rheological        property.

Rheological measurements allow for the study of chemical, mechanical,and thermal treatments, the effects of additives, the course of areaction, and can be used to predict and control numerous productproperties, performance aspects and material behavior. A particularlypreferred rheological property is viscosity and/or variation ofviscosity. The viscosity can be determined empirically using a suitableequation for Newtonian or non-Newtonian flow or from correlation using acalibration reference material which follows the same model undersimilar conditions. The measurement cycle of steps c) through f) can beconsecutively performed to provide a more continuous insight into theviscosity or viscosity change of the fluid. Preferably, steps c) throughf) are sequentially repeated under the control of a computer. Thevariation of viscosity can be affected by introduction of an oxidationgas in step c) under oxidative conditions: e.g. at a temperature fromabout 100° C. to 200° C., preferably 150° C. to about 180° C. Thevariation of viscosity can be continued until an oxidation breakdownparameter is determined, for example a relative viscosity increase (20%,25%, 50% etc), a time period, or to preset viscosity limit, etc.

Another aspect is directed to a method for measuring viscosity orrelated rheological properties of a plurality of fluid samplescomprising:

-   -   a) providing a plurality of fluid samples into individual        reservoirs, wherein the reservoirs are placed under thermostatic        control;    -   b) providing a plurality of capillary systems which provide a        flow path for the fluid sample in a reservoir, each system        having a capillary tube having a first end and a second end with        a substantially uniform diameter over a predetermined length,        the first end positionable and submerged in the fluid sample,        the second end attached to a manifold having at least one        selectable valve thereby defining a chamber of predetermined        volume, the manifold having a pressure sensor;    -   c) actuating at least one selectable valve attached to the        manifold on each capillary system to allow a gas to enter into        and pass through the manifold and capillary;    -   d) switching the actuation in step c) and suddenly inducing the        sample into the capillary by rapidly generating a dynamic        differential pressure in the chamber thus allowing the sample to        flow from the reservoir through the capillary;    -   e) detecting pressure change of the chamber as a result of the        fluid flow; and    -   f) relating the rate of pressure change to a theological        property.

A capillary system can be employed in the method of the presentinvention. Thus, this aspect is directed to a method for measuring arelative increase in viscosity in a plurality of fluid samplescomprising:

-   -   a) providing a plurality of fluid samples into individual        reservoirs, wherein the reservoirs are placed under thermostatic        control;    -   b) providing a plurality of capillary systems which provide a        flow path for the fluid sample in a reservoir, each system        having a capillary tube having a first end and a second end with        a substantially uniform diameter over a predetermined length,        the first end positionable and submerged in the fluid sample,        the second end attached to a manifold having at least one        selectable valve thereby defining a chamber of predetermined        volume, the manifold having a pressure sensor;    -   c) actuating at least one selectable valve attached to the        manifold on each capillary system to allow an oxidation gas to        enter into and pass through the manifold and capillary;    -   d) switching the actuation in step c) and suddenly inducing the        sample into the capillary by rapidly generating a dynamic        differential pressure in the chamber thus allowing the sample to        flow from the reservoir through the capillary;    -   e) detecting pressure change of the chamber as a result of the        fluid flow; and    -   f) relating the rate of pressure change to an apparent viscosity        property.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and many of the intended advantages of the presentinvention will be readily appreciated by reference to the detaileddescription in connection with the accompanying drawings.

FIG. 1 is a schematic of the alternate pressure viscometer device

FIG. 2 shows a partial exploded view of the capillary comprising asyringe

FIG. 3 show a cross sectional view of the capillary with connector

FIG. 4 is a plot of pressure verses time during a measurement cycle

FIG. 5 is a partial schematic of the alternate pressure viscometerdevice employing a positive pressure to create an increasing pressuredifferential in measurement circuit

FIG. 6 is a plot of the function of viscosity with regard to a pressurevs time relation generated on polyalphaolefin samples using thealternate pressure viscometer device

FIG. 7 is a plot of variation of viscosity in % over the test durationin hours generated by the alternate pressure viscometer device for twooils undergoing oxidation at high temperature

FIG. 8 is a nitro oxidation plot of variation of viscosity vs timegenerated by the alternate pressure viscometer device using a naturalgas engine oil base reference

FIG. 9 is an oxidation test depicting the behavior of a Viscosity IndexImprover (VII) in a binary blend with a base oil measuring the variationof viscosity vs time generated by the alternate pressure viscometerdevice

FIG. 10 is a dispersivity study plot measuring the variation ofviscosity vs time generated by the alternate pressure viscometer device

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic plan of an apparatus which can be used fordetermining a rheological property of a fluid. It is particularly suitedfor determining a change in the flow of the test fluid by repeatedevaluation of the test fluid over time and thus suitable for use inmeasuring viscosity as well as changes in viscosity. More specifically,in FIG. 1, the device 100 can be used to determine the dynamic viscosityof a test fluid 110. The device 100 includes a capillary 120 incommunication with the test fluid 110 which is contained in a reservoir112 placed in a thermostatic control system 114 used to regulate thetemperature of the test fluid to a desired set point. Commercialthermostatic control systems are widely available, particularly suitedtemperatures are from about −35 degrees Celsius to about 200 degreesCelsius. The reservoir is charged with test fluid to be examined andplaced in a thermostatic control system 114 to keep its temperatureconstant at least during the desired measurement period. Anythermostatic control system 114 can be used but it is preferred to use asystem so designed that several reservoirs 112 can be charged therein ata time, to improve efficiency of operation. The reservoir 112 can beopen to the atmosphere or sealed by a sealing member in conjunction withthe capillary 120.

The capillary 120 provides a restriction to flow path and is selected tobe a suitable length to mitigate end effects and of a cross sectionsuitable to achieve laminar flow in the region. The capillary 120 isconveniently selected as being a long thin circular tube, commonly aneedle. The capillary can also be selected such as to resemble acylindrical annulus defining an annular region between two coaxialcircular cylinders or a narrow slit formed by two narrow walls.Preferably, the capillary is a capillary tube.

The capillary 120 is connected by a manifold 130 to a selectable valve140. The capillary 120 together with the manifold 130 and the valve 140define a chamber of a predetermined volume. The volume of the chambercan be determined empirically, calculated, or by other suitable methods.In operation, the chamber receives a portion of the test fluid 110 whichflows through the capillary 120 under the influence of a difference inpressure across the capillary system. The driving pressure can bepositive pressure or vacuum, however it is important that the drivingforce be reproducible and relatively fast acting onto the chamber. Thechamber is outfitted with a pressure sensor 150 which can be used torecord the differential pressure in the chamber during a measurementcycle. The differential pressure can be output for data acquisition andcontrol and to a computing device for recordation and furthermanipulation. The selectable valve 140 can be a single valve, such asfor example a three way valve which conveniently can be in communicationwith a regulated pressure source 160, a second pressure source 170 andthe pressure gauge 150. In a preferred aspect the regulated pressuresource 160 and the second pressure source 170 are offset by more thanone selectable valve 140 such as 142 EV1 which can be a normally closedelectrovalve and 144 EV2 which can be a normally open electrovalve,wherein the electrovalves can be controlled by a data acquisition deviceand controller 180. The electrovalves are selected to be relatively fastacting valves, with valve actuation occurring in fractional seconds.

The pressure sensor device 150 converts said pressure measured to anelectrical signal, typically a voltage or current capable or beingconverted to a digital signal for processing by a data acquisition andcontroller device 180. Typically the electrical signal output by thepressure sensor is a direct current voltage, being in the order ofseveral volts. The output signal can also be direct current amperage,measured in milliamps. The pressure sensor can be used to measuredifferential pressure for example between the chamber and ambientpressure. The data acquisition and controller device 180 is used toconvert the electrical signals to digital data for further computationwith a computer 190, commonly a personal computer. Most typically, theconversion is analog to digital conversion. Modules combining dataacquisition device, a control device and a computing device arecommercially available.

The regulated pressure source 160 provides the motive force for inducinga test fluid 110 to flow through the capillary 120 and into the chamber.The regulated pressure source 160 is discontinuous in a test cycle, itis quickly applied to a predetermined setpoint to create a differentialpressure which is dynamic and changes as test fluid 110 is induced intothe chamber. Particularly preferred is a regulated reduced pressuresource 160, such as a vacuum source. Regulation of the vacuum source maybe accomplished by numerous methods known in the art. In one aspect, thereduced pressure source employs a vacuum pump 162 coupled to a vacuumtank 163 equipped with a vacuum tank pressure gauge 165. The vacuum tankis regulated around a set point by at least one vacuum tank selectablevalve 164; typical set points are from −50 millibars to −150 millibarsand have a desired precision from about +/−1 millibar around the setpoint. The vacuum tank pressure gauge 165 measures the vacuum in thevacuum tank 163, when this measure is greater than the desiredprecision, a controller 180 can open a vacuum tank selectable valve 164and optionally commence operation of the vacuum pump 162 for a period oftime until the vacuum regulation is within the desired precision. In asimilar fashion if the vacuum tank is a pressure lower than the desiredprecision, a gas can be introduced into the vacuum tank.

A second pressure source 170 is coupled to at least one selectable valve140 and used to evacuate the test fluid 110 from the capillary 120. Thesecond pressure source is regulated in flow and pressure using suitabletechniques. These parameters are not critical and selected undersuitable conditions to induce the test sample to evacuate the capillarysystem and thus are selected with reference to the regulated pressuresource 160. Typical parameters are around 0.5 bar (from about 0.1 bar toabout 5.0 bar) and around 1.0 liter/hour (from 0.1 liter/hr to about 5liters/hr). A convenient second pressure source 170 is selected from thegroup consisting of compressed gases such as air, nitrogen, oxygen,helium, NO_(x) and the like. Particularly preferred is an oxidative gasand thus the test fluid 110 can be profiled by viscosity change inreference to oxidation. During operation, the second pressure sourceflow and pressure can be adjusted to slowly bubble a gas through thetest fluid. In this method of operation, the device of the presentinvention can be used to study oxidative effects or the effects ofnitration on a test sample or a battery of test samples. Thus, forexample test samples may be evaluated for oxidative performance ornitro-oxidative performance in real time. In this aspect, the secondpressure source 170 is integrated into the operation and serves the dualfunction of evacuating the test sample from the capillary and to serveas an oxidative gas source.

FIG. 2 shows a partial exploded view of the chamber 130 which includesthe capillary cannula 120. The capillary includes a stainless steelhypodermic needle 121 which has a uniform diameter (d) over apredetermined length (l) with l>>d. On one end of the needle 121, it hasa flat tip 122 which is positional and submersed in the test fluid 110during a measurement. The size and length of the needle can be variedaccording to the expected fluid properties over the measurement. Asshown in FIG. 2, a suitable needle is for example a 25 G, 1″ long Luer.Common commercially syringe needles presented in gauge size for exampleGauge 10 to Gauge 33, with the higher number referring to the smallernominal inside diameter, preferably 15-30 G are selected. Typically, theneedle has a selected to have a length of from between about 10 mm toabout 100 mm with an inner diameter from bout 0.1 mm to about 1.5 mm.Thus suitable viscosities range from about 1 cP to about 10,000 cP. Thediameter of the capillary tube is pre-selected in accordance with thetest sample. The opposing end from the flat tip 122 has a standard Luerhub 123, which is used to connect the capillary 120 the manifold 130.The Luer hub 123 has a flanged head 124 which communicates with aconnector 125 illustrated by a male/male Luer connector. The other endof the connector 125 is attached to a similar needle 131, howeverillustrated in FIG. 2 is a 20 G, 6″ long Luer. The tip end of this isattached to a tube 136 which ultimately attaches to at least oneselectable valve 140, not illustrated. The internal volume for themanifold 130 including the capillary 120 is fixed by the selection ofthe components having the internal recesses and provides a flow path forthe test fluid 110. FIG. 3 illustrates an alternative capillary andmanifold arrangement indicating alternative connector memberconfigurations. Numerous suitable connectors and fasteners are known inthe art, including but not limited to Luer locks and Luer slip-ons,threaded connectors, connectors to tubing, etc. The connector 125 inFIG. 3 can be fabricated to have a larger internal volume for retaininga larger volume of test fluid 110 in a measurement cycle. This largervolume may also serve and prevent an aliquot of test fluid 110 fromcontaminating non-wetted areas of the manifold. It is particularlydesirable to avoid contamination of sample to the regulated vacuumsource. Also, advantageously the components which define the manifoldare inexpensive and easily replaced. Thus, for example, these componentscould be single use or readily disposable if the test sample plugsand/or contaminates the components. Commonly in oxidation tests, theoxidation by-products contaminate the capillary tubes and are notreadily cleaned.

FIG. 4 is a characteristic pressure vs time plot for a singlemeasurement of the apparatus, indicating the various pressures in thechamber during the measurement cycle. Prior to commencing the cycle thusat time t_(o) the chamber is at a pressure derived from the secondpressure source 170, P_(o) selected at a suitable pressure and flow ratesuitable to evacuate the chamber and ready the device to a measurement.A regulated pressure source is suddenly applied to provide an externaldriving force to induce a test fluid into the capillary. The time framebetween t_(o) and t_(i) is maintained to be a short duration so thatV_(o), the volume at time t_(o) can be approximated as equivalent toV_(i), the volume at time t_(i), typically the duration is fractionalseconds typically less than 0.5 and more preferably less than about 0.2seconds. The volume V_(o) is related to the designed capillary circuittotal volume. At time t_(o) the system is being readied for ameasurement, and the selectable valves 140 are suitably positioned. Forexample EV2 144 is closed and EV1 142 is opened, thereby applying theregulated pressure source 160 the system to induce the test fluid 110into the capillary 120. In FIG. 4, a reduced pressure P_(i), the drivingpressure differential related to a predetermined setpoint, is achievedand EV1 142 is closed. This rapid pressure change generates a pressuredifferential in the chamber which induces the test fluid into thechamber. The flow of the test fluid is illustrated from t_(i) to t_(f)where the pressure changes from P_(i) to P_(f) over the measurement.Typically the duration of the measurement cycle is less than a fewseconds, preferably less than 10 seconds and typically around 5 seconds.At time t_(f) indicating the end of the active measurement cycle, EV2144 is opened allowing for the test sample to chamber and ready thesystem for a new measurement cycle. An advantage of this present systemis that the measurement cycle can be repeated after time t_(f) withoutfurther intervention. Thus for example momentarily after t_(f), a newcycle can begin at a new t_(o) after only a few seconds to fractionalseconds of delay.

An alternate partial schematic configuration of the apparatus of thepresent invention is illustrated in FIG. 5. FIG. 5 employs a positivepressure driving head in the chamber 118 applied on the surface of thetest sample. The test sample is placed into a sealed reservoir 112 byusing a sealing member 128 which removably couples with the reservoirand/or the capillary 120. A further sealing member 129 cancouple/decouple the capillary 120 to the manifold 130 to allow fluidcommunication there through. The reservoir can be pressurized, wherebythe release of the pressure induces test sample 110 to flow into thecapillary 120. The release of pressure may be effectuated via a valve toa lower pressure environment as illustrated in FIG. 5; this may be theregulated pressure source 160 or the second pressure source 170. Thereservoir is placed into a thermostatic control system where temperatureis maintained. The pressure differential is measured as the test sampleflows through the capillary. The positive pressure driving head ispressure regulated 200 and controlled (not shown) to provide areproducible and discontinuous motive force in the pressure chamber 118to induce the test fluid 110 into the capillary 120. Alternatively, thepositive pressure driving head may be applied by the second pressuresource 170 through the manifold 130 and capillary 120 such that positivepressure driving head in the chamber 118 is regulated via the secondpressure source 170, the pressure regulator 200 or in combination. Thepositive pressure driving head in the chamber 118 is monitored by apressure gauge 210 and/or 150 to record the differential pressure inthis chamber during the measurement cycle. FIG. 5 illustrates the secondpressure source 170 at a driving pressure of P+Δ₁P which is offset byelectrovalves 144 and 142 from the regulated pressure source 160 and themanifold 130 which is in fluid communication with the test fluid throughthe capillary 120. The regulated pressure source 160 in this instance ispreset at P−Δ₂P. The difference in these pressures from the secondpressure source 170 and the regulated pressure source 160 in the sealedreservoir can provide positive pressure driving head in the chamber 118to induce the test sample 110 to flow through the capillary. Asdescribed above, the rate of the pressure change in the time frame isable to determine a theological property of the fluid measured.

An apparatus employed in the method aspect of the present invention maycomprise a capillary having a first end and a second end with asubstantially uniform diameter over a predetermined length, the firstend disposed for fluid communication with a liquid sample to bemeasured, the second end attached to a manifold having at least oneselectable valve, the capillary together with the manifold and the atleast one selectable valve define a chamber of predetermined volume, aregulated pressure source initially applied to induce the sample intothe capillary and generate a differential pressure in the chamber, apressure sensor attached to the chamber for outputting differentialpressure to a computing device, and a second pressure source coupled tothe at least one selectable valve for evacuating the sample from thecapillary. In a preferred operation, as the liquid sample flows throughthe capillary, the differential pressure in the chamber is dynamicchanging as the sample flows through the capillary and preferably thisis a decreasing differential pressure and thus the regulated pressuresource is a reduced pressure source for example derived from a vacuumdevice. The reduced pressure source may further comprise a vacuum pump,a vacuum tank, a pressure gauge and a control system to regulate thereduced pressure source around a defined setpoint.

In one aspect, the second pressure source is selected to be a pressureand flow rate suitable to allow the liquid sample to be measured toevacuate the capillary prior to a measurement cycle. A preferred secondpressure source is a compressed gas at a pressure greater than thepressure of the regulated pressure source. In this aspect the compressedgas is selected from the group consisting of compressed gases such asair, nitrogen, oxygen, helium, NO_(x). A particularly preferredcompressed gas is an oxidative gas.

The apparatus is particularly suited to viscosity and/or viscositychanges. Thus one aspect is directed to a capillary viscometer forsequential measurements of a liquid sample comprising: a capillaryhaving a first end and a second end with a substantially uniformdiameter over a predetermined length, the first end disposed for fluidcommunication with the liquid sample to be measured, the second endattached to a manifold having at least two selectable valves, thecapillary together with the manifold and the at least two selectablevalves define a chamber of predetermined volume, the first valve incommunication with a regulated reduced pressure source for inducing thesample into the capillary and chamber, a pressure sensor attached to thecavity for outputting differential pressure, a computing device coupledto the selectable valves and pressure sensor, and a second pressuresource at a pressure suitable to evacuate the sample from the capillary.The capillary viscometer can further comprise a device to record thevariation of pressure over a measurement. Furthermore a computing devicecan be used to process the pressure variation speed and equate to atheological property. Preferably the computing device is used to definea relationship between the pressure variation speed and viscosity usinga reference fluid.

Another aspect is directed to measuring a plurality of fluid samplesusing a single apparatus, thus disclosed is an apparatus for measuringviscosity or related theological properties of a plurality of fluidsamples, the apparatus comprising: a frame; a plurality of capillarysystems which provide a flow path for the fluid samples, each systemhaving a capillary tube having a first end and a second end with asubstantially uniform diameter over a predetermined length, the firstend positionable for fluid communication with a fluid sample containedin a sample holder, the second end attached to a manifold having atleast one selectable valve thereby defining a cavity of predeterminedvolume, the manifold having a pressure sensor; at least one pressuresource coupled to each capillary system through the selectable valve andadapted to induce the fluid sample into each capillary system at thebeginning of a measurement and to evacuate the sample at the end of themeasurement; an assembly attached to the frame for securing at least aportion of the capillary system; and a device for recording differentialpressure in each manifold and relating the differential pressure to afluid property. A preferable rheological property is viscosity.

The equations required in the calculation are derived from the followingtheoretical considerations, assuming that the product of the pressure inthe chamber and the volume of uncharged space are constant, theBoyle-Mariotte relationship can be used to determine the total volumetest fluid in the capillary and chamber at the end of the measure asillustrated in Equation 1P_(i)V_(i)=P_(f)V_(f)  (1)where P_(i) is the initial pressure at the initial time t_(i), V_(i) isthe initial volume at the initial time t_(i); and likewise P_(f) andV_(f) are the pressure and volume at time t_(f). Typically at time priorto t_(i) (for example at time t_(o)) the system is at a second pressuresource suitable to evacuate the capillary and chamber at thus the volumeis preset by design as V_(tot). From knowing the total volume of thecircuit and the pressures during the measurement, the volume of sampleinduced into the capillary and chamber can be determined. Likewise ifdesired, the flow rate of the test fluid Q could be determined fromQ=dV/dt. Particularly, by defining the flow parameters as steady-state,isothermal and laminar using a capillary of known dimensions, afunctional dependence exists between the volumetric flow and thepressure drop due to friction. From the volume of the fluid displacedand a known characteristic of the system (internal diameter and lengthof the capillary, pressure values, test duration, flow characteristics,etc) the Hagen-Poiseuille equation can be used to define the rheologicalproperties of the fluid, more specifically the viscosity, shear rate andshear stress. This equation illustrates the relationship between thevolume rate of flow and the forces causing the flow and is particularlyrelevant by systems defined for systems having a Reynolds number lessthan about 2300.

The Hagen-Poiseuille equation can be used to model the flow consideringa fluid element in the capillary tube by derivation of relationship ofthe pressure drop at the capillary as a function of capillary tubegeometry, fluid viscosity and flow rate. Accordingly, Equation 2:

$\begin{matrix}{\mu = \frac{\pi\; R^{4}\Delta\;{Pt}}{8{LV}}} & (2)\end{matrix}$where: μ is the Newtonian apparent viscosity, R is the radius of thecapillary, L is the length of the capillary, t is time measured of theinterval, ΔP is the pressure measurement over the interval and V isvolume measured over the interval. In a similar fashion the shear stressand the shear rate can be determined. The shear stress at the tube wallcan be obtained as illustrated in Equation 3 with the shear rateillustrated in Equation 4:

$\begin{matrix}{\tau_{w^{\prime}} = \frac{R\;\Delta\; P}{2L}} & (3) \\{\gamma_{w} = {\frac{4Q}{\pi\; R^{3}} = \frac{4V}{\pi\; R^{3}}}} & (4)\end{matrix}$where τ_(w) is the shear stress at the capillary wall, γ_(w) is the wallshear rate, Q is the volumetric flow rate.

In a more pragmatic way, a function between the dynamic viscosity (cP)and the variation of the pressure vs time (mb/s) relationship can bedefined using a set of adapted calibration products. Known viscosity ofreference samples can be used to calibrate the system under specific setof conditions. Using the device of the present invention the pressurevariation speed (P_(v)) can be measured and stored by a data acquisitiondevice for use in defining a relationship between the pressure variationspeed and the viscosity. Numerous mathematical models can be employed todescribe this relationship, such as using the method of least square tofit the curve to define μ=f(P_(v)) or using geometric mean values, etc.The generated coefficients are stored, typically in a computer, andaccessed to define the viscosity of an unknown sample from thisrelationship. Even without calibration, the relative viscosity changecan be determined using repeat measurements.

For Newtonian fluids the viscosities are independent of shear rate, thusγ_(a)=ατ_(w) or γ_(w)=μτ_(w). For non-Newtonian fluids, the viscositiesof the test samples will vary with shear rates. Numerous acceptablemodels have been used to define these behaviors. For example, for powerlaw fluids, the apparent shear rate is related to the shear stress byγ_(α)=(ατ_(w))^(1/n) and thus μ=(4n/(α(3n+1))_(n)(γ_(a))^(n−1) andγ_(w)=α((3n+1)/4n)τ_(w) ^(1/n) where n is a power law exponent. If theliquid behaves as a Bingham fluid, the apparent shear rate is related tothe shear stress by γ_(α)=α(τ_(w)−τ_(B1)) where τ_(B1) is given from theyields stress relation τ_(r)=(3/4)τ_(B1) and thus μ=1/α andγ_(w)=α(τ_(w)−3β/4). If the liquid behaves as a Casson fluid, theapparent shear rate is related to the shear stress by γ_(a)^(1/2)=α(τ_(w) ^(1/2)−τ_(CA)) where τ_(r)=(49/64)τ_(CA) ² and thus μ=a²and γ_(w) ^(1/2)=(τ_(w) ^(1/2)−τ_(CA))/α, where τ_(CA) is the Cassonyield stress.

In the study of lubricants and lubricating oils, oxidation is animportant phenomenon that needs to be controlled to increase oil drainintervals for engine oils and maintain good lubrication of the engineduring the whole length of the drain interval. Many engine tests havebeen and are being developed to reproduce this phenomenon and qualifyengine oils with adequate performance. These engine tests are long,expensive and require big amounts of test lubricant, which produce a lotof waste.

In order to classify the performance of new lubricants, additives, andformulations; it has been imperative to develop improved laboratorytests for predicting and simulating actual engine tests response moreexpeditiously with improved response, in less time and with less sample.Numerous laboratory oxidation tests have been developed for many yearsfor this purpose, but even though they use less amount of test oil, theystill require several days of testing, use around 100 g of test oil andrequire regular sample taking to be able to follow the oxidationreaction over time. Most of the existing oxidation tests requiresampling to be able to follow the evolution of key parameters such asviscosity, oxidation, and/or nitration IR peaks, TAN, metalsconcentration and the like. Removal of the sample for quantificationduring the test is difficult and changes concentration in the bulksample, thus many methods only take a sample at the end of the test.Some other methods follow the evolution of oxygen pressure in a closedreactor (rotary bomb oxidation test, TFOUT, etc.). Typically theinhibition period is determined from measurement of oxygen absorption(e.g. in an oxidation bomb apparatus), heat of reaction (e.g. DSC),formation of reaction products (e.g. acids, peroxides, insolubles,gaseous products) and/or change in physico-chemical properties such asincrease in viscosity. However, a common feature to these tests is theneed for big and costly equipment and the need for handling the samples,the method aspect of this invention overcomes this problem. An aspect ofthe method of the present invention can be used to screen and/orevaluate the antioxidant capabilities of base oils, engine oils, andlubricating oil additives by determining the relative viscosity increasein the presence of oxidation conditions.

The alternate pressure viscometer of the present invention can beemployed to provide a rapid analysis of the relative increase ofviscosity in a myriad of test conditions, such as high temperatureoxidation/nitration curves, dispersivity study, thermic shear of VIIetc. One feature of the device of the present invention is that it isable to measure in-situ the oil viscosity during an oxidation andprovide viscosity measurements in-succession and without undueinterruption since the measurement cycle is automated and short.

The alternate pressure viscometer of the present invention can also beemployed to quickly provide the Viscosity Index (VI) of a liquid usingat least two different temperature reference points. Petroleum oils havedifferent rates of change of viscosity with temperature. The VI is amethod of representing this change, based upon comparison with therelative rates of change of two arbitrarily selected types of oil thatdiffer widely in this characteristic. A high VI indicates a relativelylow rate of change of viscosity with temperature. Conversely, a low VIindicates a relatively high rate of change of viscosity withtemperature. A method for measuring the VI of an oil is described inASTM D 2270 incorporated herein by reference. Typically the VI scale isan empirical one based upon the arbitrary assignment of VI values to twodifferent crude oils, commonly by measuring kinematic viscosity at 40°C. and 100° C. The viscosity temperature relationship of a Pennsylvaniacrude was arbitrary assigned a VI of 100 and the same relationship of aGulf Coast crude was assigned a value of 0. The device of the presentinvention can be utilized to measure an oil sample at two controlledtemperatures for example at 40° C. and 100° C., determine kinematicviscosity from the measured pressure variation rate, as described hereinabove, and calculating the Viscosity Index using data from publishedtable that are stored in a computer or by extrapolating such data.

EXAMPLES

The following examples were performed to demonstrate the performance ofthe device of the present invention.

Example 1

This example illustrates operation of the device using a singlecapillary system depicted in FIG. 1 and discloses a method forcalibration of the apparatus using samples of known viscosity togenerate a correlation between the pressure difference vs time measuredin chamber as a function to the samples viscosity. The method indicatedwas used to calibrate a single capillary system, if a multi channelcapillary was used; the calibration would be performed for each channel.Additionally, if the operational conditions were modified, for example,if the same capillary system was to be used with different values ofreduced pressure, or measurement duration, or temperature, it would benecessary to calibrate again the system taking account for this new setof parameters.

In this example, four petroleum oils were selected which wererepresentative of the range of viscosity chosen to study. Moreparticularly four polyalphaolefin (PAO) oils were chosen: PAO2, PAO4,PAO5 and PAO7. Polyalphaolefins are manufactured by the oligomerizationof linear alpha olefins (commonly 1-decene or 1-dodecene) followed byhydrogenation to remove unsaturated bonds and fractionation to obtainthe desired product slate. PAO's are commonly categorized by numbersdenoting the approximate viscosity in centistokes at one hundred degreesCelsius. The Kinematic viscosities (cSt) at 100° C. of these productswere measured with a Hubbelhod viscometer according to the ASTM D445 andtransformed in dynamic viscosity (cP) by using the density of each oil(cP=cSt*density).

The capillary system employed was similar to the capillary systemdepicted in FIG. 2. Here we use a system made with a Luer needle gauge25 G-1″ as the capillary, a male/male Luer splice and a second Luerneedle gauge 20 G-6″. This latter needle is spiced to the electrovalvecircuit with 12 cm of a silicone tubing (internal diameter 0.5 mm), seeFIG. 2. The 25 G-1″ needle was selected to measure viscosities betweenaround 1 cP and 10 cP. If other ranges of viscosity are to be studied,the needle and measure parameters may have to be modified. The capillaryneedles have generic length, for the range 8-25 cP and 3,000-10,000 cPthe standard needle was cut to obtain the desired length; the value ofthe corresponding capillary length is indicated from the measure of thetip portion. The area between 25 cP and 3,000 cP was not included in theinitial study but sets of parameters could easily be determined. TABLE 2gives a set of parameters for three ranges of viscosities studied inthis example, namely: 1 to 10 cP, 8 to 25 cP and 3,000 to 10,000 cP.These values were chosen for convenience to test the invention overvarious ranges of viscosity; these parameters are not intended to limitthe invention or the working scope of the invention. For example, for agiven range of viscosity with the other parameters being equal, acapillary having a smaller internal diameter of the needle will likelylead to a higher driving pressure and/or longer period for the sample.Numerous values and parameters can be defined and optimized.

The viscosity ranges and parameters set forth in this example allowquick access to study the flow characteristics and allows formulator tostudy simulated flow regimes encountered in monograde and multigradeengine oils according to the ASTM D445 and ASTM D5293 requirementsneeded in the SAE-J300. This kind of formulation data often requiresthree, four or more pre-blends before reaching the right viscosities,and even if the last measures have to be done in ASTM conditions, a lotof time can be gained by using this invention for the pre-blends. Forexample, using the device of the present invention approximately 6minutes including the temperature stabilization is sufficient to get anaverage of three or four values. This compares to around 15 minutes fora single measure with the Cold Cranking Simulator (ASTM D5293) or around20 minutes for a Hubbelhod viscometer (ASTM D445).

The device was set-up using an apparatus as illustrated in FIG. 1wherein the sample to be tested was placed in a reservoir (open toambient air) and placed into a temperature control bath. The capillarysystem was positioned so that the tip of the needle was below thesurface of the sample. A 0.5 bars regulated compressed air source wasattached, and the flow rate of 1 liter per hour allows bubbles to emergefrom the tip of the syringe and to evacuate any sample. The reducedpressure motive force was secured by using a vacuum pump controlled andcoupled to a 5 liter vacuum tank, the vacuum set point was set at −110millibar+/−0.5 millibar and the test measurement cycle was set at 5 sec.The reduced pressure source was controlled by electrovalves. Theelectrovalves used in the system are solenoid-operated 2 or 3-way pinchvalves, 24 VDC, coupled to the chamber using silicon tubing with aninternal diameter of 0.8 mm ( 1/32″). A quick acting valve is desirable;the response time for these EV's is between 5 and 20 msec. The relativepressure gauges used for the reduced pressure control and the measure ofthe pressure variation speed have a range of 0 to −300 mbar for ananalogical output of 0-10 VDC.

Software and hardware for the control of the process and the signalacquisition are from National Instrument Company (Austin, Tex.):LabVIEW™ for the software and field point modules for input/outputsignal, with the final data sent to an Excel spreadsheet. The process islargely automatic; the operator places the oil in the reservoir, thereservoir is placed in the thermostatic bath and starts the process. Atthe end of the process, the reservoir has to be removed and cleaned. Anauto-sampler could easily be used to make measurements one after theother automatically for number of products in different reservoirs.Commonly robotic samplers can be fitted with one or more capillarysystems to sample a single reservoir or to multiple reservoirs, eithersimultaneously or in rapid serial mode. Numerous robotic samples areknown in the art and commercially available, suitable three axis robotsare disclosed for example in U.S. Pat. Nos. 5,476,358 and 5,234,163.According to the capillary used and range of viscosity studied, thesoftware program allows modifying the set point for the pressure, thetime for applying the reduced pressure, the measurement duration, thetime between two measurements, and the coefficients of the equationμ=f(P) explained below.

Measure: approximately 3 milliliters of PAO were placed in an open ended(to ambient air) reservoir which was allowed to equilibrate in aconstant temperature silicone bath (or dry aluminum bath) at 100° C.degrees Celsius. The capillary was positioned to have at least one endimmersed into the fluid, and then the flow rate of air was set to assurethat air was bubbling in the PAO. The system was allowed to equilibratefor a period of time until the oil temperature in the reservoir isstabilized at 100° C. Due to the small quantity of oil in the reservoir,typically around 5 minutes are sufficient. To begin the measurementcycle, the regulated reduced pressure is quickly applied to thecapillary and chamber to allow sample to begin to flow into thecapillary tube. The reduced pressure is applied by action on the adaptedelectrovalve during 0.3 seconds and during the following 5 seconds thepressure variation over time is recorded. The acquisition rate is fixedto 10 values per second, the 50 values recorded look like a light curve;the computer is employed to process a linear regression on these 50values, the slope of this line is used to define the mean of thepressure variation speed (P_(v)) in mb/s. After approximately 5 secondsof measure, compressed air is applied to empty the viscometer. Typicallyonly a few seconds are required before the air bubbles again in the oil,for example here we waited approximately 10 seconds. To increase theprecision of the test we repeated the measurement cycle three times andthe average of the three pressure variation speeds was calculated. Thereservoir was removed from the bath and the parts were rinsed with asolvent like heptane and dried before the next oil is measured accordingto the same procedure describe above.

After measuring the four PAO samples, the viscosities (μ) and thePressure variation speed (P_(v)) are stored. Using this data andmathematically manipulating for example using the method of leastsquares, the equation μ=f(P_(v)) is defined (TABLE 1 and FIG. 6). Thecoefficients of this equation are stored in the computer and are thenused in the program to directly give the viscosity of an unknownproduct.

TABLE 1 calibration for a viscosity range 1 to 10 cP Pressure variationViscosity μ speed (mb/s) (cP) PAO2 18.56 1.760 PAO4 13.36 4.1 PAO5 11.365.300 PAO7 9.72 7.000 μ = 0.0427P² − 1.7914P + 20.301

Example 2

This example is a presentation of some sequences of measures done onseveral petroleum products. The data demonstrates the repeatability ofthe device of this invention when it is employed to measure viscosity.These measures have been done with the single capillary system andaccording to the procedure and calibration method described inExample 1. The specific parameters for the capillary systems employed inthis example are set forth in Table 2.

TABLE 2 Set of parameters Viscosity range 2 to 8 to 3000 to 10 cP 25 cP10000 cP Internal diameter of the needle (mm) 0.26 0.41 0.84 (25 G) (22G) (18 G) Length of the needle 1″ 10 mm 10 mm Starting pressure (mb)−110 −90 −220 Measure duration (s) 5 2 5

The device and parameters were not optimized; accordingly the values ofrepeatability described below have to be considered as the minimum ofperformance for the ability of this invention. Repeatability has beendemonstrated for two ranges of viscosity 1-10 cP and 3,000-10,000 cP.

Results:

-   -   Range 1-10 cP    -   Four petroleum base oils have been chosen, which belong to the        four main groups defined by API and ATIEL BASE STOCK CATEGORIES        and cover the studied viscosity area:        -   PAO 2: Polyalphaolefin GROUP-4        -   Chevron RLOP 100N: Mineral oil GROUP-2        -   Total 150N: Mineral oil GROUP-1        -   Chevron UCBO-7R: Mineral oil GROUP-3            20 measures have been done sequentially for each product            with the same parts and the same procedure described in            Example 1. TABLE 3 shows the results and the statistics            evaluation.

TABLE 3 Results and statistics data range 1-10 cP GROUP-3 Range GROUP-4GROUP-2 GROUP-1 UCBO 7R 1-10 cP (100° C.) PAO2 (cP) 100N (cP) 150N (cP)(cP)  1 1.780 3.932 5.592 6.980  2 1.769 4.008 5.436 6.953  3 1.7573.966 5.448 6.977  4 1.787 3.958 5.439 6.948  5 1.748 4.043 5.445 6.902 6 1.777 3.967 5.437 7.013  7 1.769 4.002 5.442 6.859  8 1.774 4.0215.413 6.927  9 1.784 4.018 5.464 6.852 10 1.788 4.024 5.548 6.953 111.756 4.056 5.433 6.926 12 1.774 4.015 5.425 6.972 13 1.766 4.066 5.4466.902 14 1.776 3.957 5.565 6.904 15 1.800 3.983 5.429 6.840 16 1.7463.986 5.469 6.899 17 1.777 4.025 5.537 6.945 18 1.789 4.002 5.471 7.00119 1.804 3.936 5.451 6.983 20 1.751 3.986 5.473 6.982 Average 1.7743.998 5.468 6.936 Degrees of freedom 19 19 19 19 (dof) Variance 0.0002680.001414 0.002581 0.002490 Standard deviation 0.0164 0.0376 0.05080.0499 K student 2.093 2.093 2.093 2.093 (0.025 − dof = 19)Repeatability (single 0.048 0.111 0.150 0.148 measure) (cP) Repeat.(average of 0.034 0.079 0.106 0.104 2 measures) (cP) Repeat. (average of0.028 0.064 0.087 0.085 3 measures) (cP)

As used herein the following terms are defined to mean:

“Repeatability (single measure)” means: The difference between twosingle measures done on the same product in the same conditions by thesame operator will be different of 0.048 cP (example for PAO2) only inone case out of twenty.

“Repeat. (Average of 2 measures)” means: The difference between theaverage of two measures done on the same product in the same conditionsby the same operator will be different of 0.034 cP (example for PAO2)only in one case out of twenty.

“Repeat. (Average of 3 measures)” means: The difference between theaverage of three measures done on the same product in the sameconditions by the same operator will be different of 0.028 cP (examplefor PAO2) only in one case out of twenty.

Range 3,000-10,000 cP

-   -   One petroleum base oil, a Bright Stock Solvent, was measured 20        times at 5 different temperatures (2.5° C., 5° C., 7.5° C.,        10° C. and 12.5° C.) to cover the range of viscosities between        3,000 cP and 10,000 cP.

TABLE 4 Results and statistics data, range 3,000-10,000 cP Range BSS BSSBSS BSS BSS 3,000-10,000 cP 2.5° C. 5° C. 7.5° C. 10° C. 12.5° C.  110944 8790 6651 4854 3153  2 11058 8859 6715 4785 3163  3 10994 88096740 4807 3186  4 10960 8753 6785 4801 3200  5 10994 9137 6777 4775 3184 6 10978 8886 6851 4785 3192  7 10932 8835 6737 4835 3180  8 11059 89576753 4844 3173  9 10963 8982 6646 4771 3186 10 11170 8974 6702 4838 318111 10799 8926 6645 4870 3224 12 10971 8733 6601 4837 3185 13 10984 88396808 4718 3180 14 10714 8889 6651 4843 3178 15 10853 9035 6715 4831 318816 10914 8926 6740 4814 3216 17 11127 8673 6785 4839 3222 18 10934 86966777 4862 3197 19 10922 8586 6851 4893 3200 20 10883 8994 6737 4818 3209Average 10958 8864 6733 4821 3190 Degrees of freedom 19 19 19 19 19(dof) Variance 10704 18189 4783 1631 335 Standard deviation 103 135 6940 18 K student 2,093 2,093 2,093 2,093 2,093 (0.025 − dof = 19)Repeatability (single 306 399 205 120 54 measure) (cP)) Repeat. (averageof 217 282 145 85 38 2 measures) (cP) Repeat. (average of 177 230 118 6931 3 measures) (cP)

Example 3

Oxidation Test

The following study has been done with an eight channel capillarysystem. It differs from the single channel capillary system in the sensethat the reduced pressure part is common for the eight capillaries sothe measures have to be done one after the other. At the opposite, thegas pressurized part is unique for each cell and thus, there was arequirement to splice eight flowmeters. Here we used 150 mm flowmeterswith a stainless steel ball for a range of 0 to 5 Liter per hour. Thegas used for this example is air.

The oxidation performance of two passenger car engine oils is studiedhere. Each oil has been tested four times. A sequence of the eightmeasures of viscosity is done every 5 minutes. From the recorded valueswe calculate the percent of variation of the viscosity. The test is doneat a temperature of 180° C., the air was bubbled in the oil at aflowrate of 1 l/h to commence suitable oxidation conditions. The test isstopped when the relative viscosity increase reached around 20 percent.TABLE 5 gives a summary of the parameters for this example.

TABLE 5 Example 3 parameters Oil quantity 3 ml Catalyst no Gas air Gaspressure 0.5 b Gas flow 1 l/h Temperature 180° C. Reduced pressure −110mb Internal diameter of the needle 25 G Length of the needle 1″ Time toapply vacuum 0.3 s Measure duration 5 s Time between two serial of eightmeasures 5 mn

FIG. 7 shows the eight recorded curves demonstrating the oxidation curvegenerated for reference oil A and reference oil B illustrated in a plotof the % of variation of viscosity plotted against the cumulative timeof the test.

Example 4

We used the same device as configured in Example 3; however a differentsecondary pressure source was used to illustrate the ability to use gasother than air for an oxidation test. Here, NO₂ was selected asoxidative gas to test the nitro-oxidation performance of Natural GasEngine Oils (NGEO). Two internal low and high NGEO reference oils aretested. Oil A is the low reference and Oil B is the high reference. Onlytwo capillaries have been used for this test, the six remainingcapillary systems were disconnected. TABLE 6 gives details of parametersfor this example.

TABLE 6 Example 4 parameters Oil quantity 3 ml Catalyst 1: Naphtenate ofiron 75 ppm Catalyst 1: Naphtenate of copper 75 ppm Gas NO2 Gas pressure0.5 b Gas flow 2 l/h Temperature 150° C. Reduced pressure −110 mbInternal diameter of the needle 25 G Length of the needle 1″ Time toapply vacuum 0.3 s Measure duration 5 s Time between two serial of eightmeasures 5 mn

FIG. 8 shows the kind of curve we can record. An advantage demonstratedby this plot and during the measurement cycle is the real timeobservation of the behavior of products during oxidation tests.

Example 5

This example depicts an oxidation test. Here we investigated thebehavior of a Viscosity Index Improver (VII) at 180° C. in a binaryblend with a base oil. The VII selected in this test is a hydrogenatedstyrene isoprene and a petroleum Group 3 base oil. We include in thisbase oil the necessary quantity of VII to obtain a viscosity of 14 cStat 100° C. By using two of the eight capillary devices described inExample 3, we record the viscosity of the base oil alone in the firstcell and the blend of base oil and VII in the second cell. The test isdone at a temperature of 180° C. TABLE 7 gives details of parameters forthis example.

TABLE 7 Example 5 parameters Oil quantity 3 ml Catalyst No Gas Air Gaspressure 0.5 b Gas flow 1 l/h Temperature 180 ° C. Reduced pressure −110mb Internal diameter of the needle 25 G Length of the needle 1″ Time toapply vacuum 0.3 s Measure duration 5 s Time between two serial of eightmeasures 5 mn

FIG. 9 shows a very rapid drop of the viscosity after around three hoursof test which approaches the viscosity of the base oil after about 6hours.

Example 6

Recording the variation of the viscosity versus time can show otherperformance attributes than oxidation. In this example we study thebehavior of a dispersant additive. The role of a dispersant additive inengine oil is to maintain in suspension the carbon and other smallparticles which can appear in the oil. This additive protects the engineagainst plugging.

In the same procedure illustrated in Example 3 but at a temperature of110° C. and using nitrogen (1 liter per hour) to prevent against anyoxidation effect, we compared the viscosity variation of petroleum baseoil, TOTAL 330Neutral (Group 1), where 2% of carbon black is well mixedbefore the test to the same oil where 2% of dispersant additive comingfrom the alkyl succinimide family is added. The introduction of carbonblack in the oil increases the viscosity. If the carbon black remains insuspension in the oil, the viscosity remains at the same level. When thecarbon black settles from suspension, the viscosity will decrease.

TABLE 8 gives details of parameters for this example. A differencebetween the procedures described above is the quantity of oil (5 mlversus 3 ml). The larger sample is due in part because the extremity ofthe capillary where N₂ is bubbled should be positioned approximately inthe middle of the height of the oil in the reservoir and not at thebottom where carbon black decants. This positioning is to preventagainst plugging the needle during a sample sequence.

TABLE 8 Example 6 parameters Oil quantity 5 ml Catalyst no Gas N2 Gaspressure 0.5 b Gas flow 1 l/h Temperature 110° C. Reduced pressure −110mb Internal diameter of the needle 25 G Length of the needle 1″ Time toapply vacuum 0.3 s Measure duration 5 s Time between two serial of eightmeasures 5 mn

FIG. 10 shows the rapid decrease of the viscosity of the oil which doesnot contain the dispersant. For the blend which contains the dispersant,the viscosity is constant for a long period of time with a decreaseappearing after 12 hours.

1. A method for screening or determining a variation in viscosity of afluid, the method comprising: a) providing a fluid sample to a reservoirplaced in a thermostatic control system; b) placing a capillary in fluidcommunication with the fluid sample, wherein the capillary has a firstend and a second end with a substantially uniform diameter over apredetermined length, the first end being submerged in the fluid sampleto be measured, the second end attached to a manifold having at leastone selectable valve, the capillary together with the manifold and theat least one selectable valve define a chamber of predetermined volume;c) actuating at least one selectable valve attached to the manifold toallow an oxidative gas to enter into and pass through the manifold andcapillary; d) inducing the sample into the capillary by rapidlygenerating a dynamic differential pressure in the chamber thus allowingthe sample to flow from the reservoir through the capillary; e)detecting pressure change of the chamber as a result of the fluid flow;and f) relating the rate of pressure change to the variation inviscosity.
 2. The method of claim 1, wherein the viscosity is calculatedfrom the Hagen-Poiseulle relation according to equation 2:$\begin{matrix}{\mu = \frac{\pi\; R^{4}\Delta\;{Pt}}{8{LV}}} & (2)\end{matrix}$ wherein μ is the apparent viscosity, R is the radius ofthe capillary, L is the length of the capillary, t is time measured ofthe interval, ΔP is the pressure measurement over the interval and V isvolume measured over the interval.
 3. The method of claim 1, wherein theviscosity is calculated from a stored correlation of the pressurevariation speed using at least two calibration fluids having knownviscosities.
 4. The method of claim 3, wherein the fluid sample is anon-Newtonian fluid.
 5. The method of claim 1, wherein steps c-f aresequentially repeated under control of a computer.
 6. The method ofclaim 1, wherein the thermostatic control system is maintained at atemperature between about 100° C. and about 200° C.
 7. The method ofclaim 5, further comprising determining an oxidative breakdownparameter.
 8. A method for measuring a relative increase in viscosity ina plurality of fluid samples comprising: a) providing a plurality offluid samples into individual reservoirs, wherein the reservoirs areplaced under thermostatic control; b) providing a plurality of capillarysystems which provide a flow path for the fluid sample in a reservoir,each system having a capillary tube having a first end and a second endwith a substantially uniform diameter over a predetermined length, thefirst end positionable and submerged in the fluid sample, the second endattached to a manifold having at least one selectable valve therebydefining a chamber of predetermined volume, the manifold having apressure sensor; c) actuating at least one selectable valve attached tothe manifold on each capillary system to allow an oxidation gas to enterinto and pass through the manifold and capillary; d) switching theactuation in step c) and suddenly inducing the sample into the capillaryby rapidly generating a dynamic differential pressure in the chamberthus allowing the sample to flow from the reservoir through thecapillary; e) detecting pressure change of the chamber as a result ofthe fluid flow; and f) relating the rate of pressure change to anapparent viscosity property.
 9. The method of claim 8, wherein thedynamic pressure differential is decreasing with fluid flow.
 10. Themethod of claim 9, wherein steps c-f are sequentially repeated undercontrol of a computer.